The readings for the humidity and pressure can be off because they depend on the temperature reading. There is a instance variable t_fine that both humidity and pressure reference. It is only initialized in when reading the temperature. This fact should be documented.

I propose adding a single method that reads all three measurements in a single call to the sensor. In my measurements on a Moteino M0, the time required to read temperature, humidity and pressure can be reduced by about 35% (1139 us vs 1729 us). Reading all values in a single call ensures the sensor values are all from the same measurement.

My first prototype looks like this. However, I am thinking on changing this a bit to align it better with the style of BME280 class with various methods (ie temperature in C / F, altitude in meters / feet). Thoughts?

struct BME280_SensorMeasurements
{
  public:
	float temperature;
	float pressure;
	float humidity;
};

class BME280
{
  public:
    void read(BME280_SensorMeasurements *measurements);
}

edit: fixed units from ns to us.